This invention relates to a switching arrangement for telecommunications systems, more particularly telephone systems, for identifying one of two or more subscriber sets connected over individual branch circuits to a common trunk leading to an exchange.
Arrangements such as that described above are commonly called party lines and are employed generally with a view to economizing on subscriber lines in thinly populated areas. The subscribers have the possibility of cutting into calls in progress to initiate emergency calls or conduct internal conversations. Due to the introduction of automatic long-distance traffic, there exists a demand for individual call data recording and, thereby, the necessity for clearly identifying the connected subscribers. A commonly used technique for solving this problem is to identify the individual subscriber sets through individual identification frequencies and to provide them with means for transmitting and/or receiving these frequencies.
Other prior art arrangements employ the current-direction dependency of electrical rectifiers for the identification of the subscriber sets. By using a ground wire, up to four subscriber sets can be distinguished from one another. However, the use of ground wires in the subscriber sets results in a great many disadvantages.
In another prior art arrangement there are looped into the individual branch circuits non-linear circuit elements which have operating points having a low differential resistance for both current directions. These operating points, however, cannot be attained for one current direction until a threshold voltage has been exceeded. One disadvantage of such circuit elements which, e.g., may be realized by means of four-layer diodes, is that special firing or triggering processes must be performed.
Furthermore, at first only two groups of subscriber sets must be distinguished immediately, the full identification being performed in accordance with the staggered values of the threshold voltages of the inserted circuit elements by gradually increasing the direct voltage for identification. Moreover, without special provision one cannot intrude in a call in progress, because in such a connection the voltage between the wires of a common trunk decreases to such a level that a second four-layer diode can no longer be fired.
In still another prior art arrangement, the non-linear circuit elements looped into the individual subscriber branch circuits exhibit such a currentdirection-dependent threshold behavior that their differential resistance is positive at all times and possesses a comparatively high value below the threshold voltage and a relatively low value thereabove. The threshold voltage of the circuit elements, which may be realized through parallel connection of rectifiers having opposite polarities or by connecting in series Zener diodes having opposite polarities, must be graded in descending sequence relative to the circuit elements following one another according to their ordinal numbers.
In this case, various partial voltages must be made available for purposes of identification, which have a well-defined voltage value relative to the individual threshold voltages of the circuit elements. The subscriber set to be identified is ascertained through stepwise application of the voltages.
It is, therefore, an object of the invention to provide apparatus which simplifies the aforementioned identification process in the exchange and which minimizes the influence of parasitic voltages, while keeping low the number of individual subscriber sets.
A further object of the invention is to provide such apparatus which will permit the individual subscribers to continue to have the possibility of intervening in a call in progress.